Nowadays, image scanners have become common peripheral equipment of computers. As known, resolution of an image scanner is one of the important indices to evaluate a scanner. In general, resolution consists of two components, i.e. horizontal resolution and vertical resolution. The horizontal resolution depends on the specification of the optical system, the pixel number of the image pickup device such as a charge-coupled device (CCD) and/or hardware circuitry. The vertical resolution, on the other hand, relates to the operations of the transmission device 11 and step motor 12 shown in FIG. 1.
An advanced image scanner provides two options for users. The image scanner may perform a scanning operation on the conditions of a relatively high speed but relatively low resolution. Alternatively, the scanning operation may be performed on the conditions of a relatively high resolution but a relatively low speed. The prior art technique adjusts the scanning resolution and speed by changing the revolving speed of the step motor. However, the motor at a high revolving speed generates significant noise, and is required to be a high torque motor that is of high cost and bulky.